Step 11: Hold on.. "brightness mapped"?

Step 12: Man, that was a lot of text

Paste the bright and dark stock images into the composite as separate layers. After desaturating the stock images, perform an edge detect (Filters - ...

Step 8: The light layer

This process is almost exactly the same as adding the dark layer but with a few notable exceptions. Open the light stock image, paste it into a new layer in the composite and desaturate the stock image as before.

Now, the desaturated image is not quite ready for being used as a layer mask. Remember the white parts make the image opaque, and black makes it transparent. If you used this as a layer mask, it would use the bright parts of the bright image and the dark parts of the dark image- the exact opposite of what we want. To rectify this, invert the greyscale image (Layer -> Colors -> Invert). If your image needs brightening, do the curves operation as before- mine didn't, so I skipped this step.

Copy the bright layer mask image, create a layer mask on the light layer and paste the bright layer mask image into it as before. This should make the brighter parts of the composite image mroe vibrant- it usually has the effect of making daytime sky more vivid blue, increasing the colour gradient and decreasing wash-out.

I would love to be nice about these directions, but I cannot be constructive because I did not find them useful. I have effectively produced an image worse than the 3 I started out with using your directions, because they are not clear.

I wrote this Instructable ages ago, and coming back to it I sometimes think "How on earth did I write so much about such a simple process?" I'm probably going to strip out and rewrite the instructions, in a simpler manner.

These instructions are great for someone who is familiar with GIMP. Thank you. I think you should state that upfront, so people have their expectations set right. Thanks for taking the time. I found it very usefull.